Field of the Disclosure
This application relates generally to flyback converters and more specifically the invention relates to zero voltage switching (ZVS) flyback converters.
Background
Switch mode power converters are widely used for household or industrial appliances that require a regulated direct current (dc) source for their operation, such as for example battery chargers that are commonly used in electronic mobile devices. Off-line ac-dc converters convert a low frequency (e.g., 50 Hz or 60 Hz) high voltage ac (alternating current) input voltage to a required level of dc output voltage. Various types of switch mode power converters are popular because of their well regulated output, high efficiency, and small size along with their safety and protection features.
Safety requirements for isolated switch mode power converters generally require the use of high frequency transformers to provide galvanic isolation between the inputs and outputs of the switch mode power converters in addition to the voltage regulation at the output. Popular topologies of isolated switch mode power converters may include flyback, forward, isolated half/full bridge, among many others including resonant types.
One source of loss in switch mode power supplies is the switching loss during turn on and turn off of the power switch. In an isolated switch mode power converter High Frequency (HF) turn off oscillations may happen due to resonance between the leakage inductance of the transformer and the output capacitance of the power switch (e.g., total parasitic capacitance across the power MOSFET). In discontinuous current mode DCM operation of an isolated switch mode power converter, in addition to above mentioned HF turn off oscillations, a second lower frequency oscillation may also happen between the magnetic inductance of the transformer and the output capacitance of the power switch. This second lower frequency oscillation may often be referred to as relaxation ringing.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.